Hydrogen generators have long been used to generate hydrogen through the hydrolysis of chemical hydrides, and in particular, metal hydrides. For example, U.S. Pat. No. 2,334,211 discloses a hand-held generator containing calcium hydrides which, when submersed in water, produces sufficient hydrogen to fill an emergency signal balloon. More recently, the most common portable source of hydrogen is hydrogen bottles or tanks in which the hydrogen is stored under pressure. The hydrogen stored in these bottles or tanks is generated at a hydrogen production plant, shipped as a cryogenic liquid, vaporized, and expanded into the tanks or bottles under pressure. These hydrogen tanks or bottles are generally bulky and rather heavy. Further, when a tank or bottle is exhausted, it must be replaced with another tank or bottle. Storage tanks or bottles are utilized in field applications because, typically, hydrogen production facilities have been considered too large, too heavy, too expensive and in many instances, too unsafe, for portable operation. In response, there have been attempts to develop practical and portable hydrogen generators.
One such portable hydrogen generator, for example, is disclosed in U.S. Pat. No. 4,155,712. U.S. Pat. No. 4,155,712 discloses a small portable hydrogen generator utilizing a metal hydride and water vapor in which hydrogen can be automatically produced on demand or at a constant pressure feed over widely varying hydrogen demand rates without water supply contamination or metal hydride caking complications. Among the problems in the use of demand responsive hydrogen generators, however, is that of a sudden requirement made on the water vapor such that water instead of the water vapor could be drawn into direct contact with the fuel, thus causing a malfunction. U.S. Pat. No. 4,261,955 addresses this problem by utilizing a wall means for separating adjacently disposed solid fuel and water compartments. The wall means includes two spaced apart porous hydrophobic membranes. During normal production of hydrogen gas, the membranes are of a character as to normally only pass water vapor from the water supply to the fuel compartment. If an abnormal demand is made on the water vapor, it could inadvertently cause unvaporized water to pass through one of the membranes. Therefore, a hydrogen gas outlet is positioned between the spaced-apart membranes to pull off the water before it could reach the metal hydride fuel.
Both U.S. Pat. Nos. 4,155,712 and 4,261,955 disclose using compounds with a chemical hydride, such as lithium aluminum hydride (LiAlH.sub.4) in an attempt to control internally generated heat. Presently, however, no commercially viable small portable hydrogen generators are able to supply hydrogen instantaneously and sustain a constant flow of hydrogen while controlling external structural heating and uncontrollably escalating temperatures and pressures in the generator due to uncontrolled hydrogen release by the chemical hydride. As a result, industry typically still uses high pressure gas storage, metal hydride storage, or liquid hydrogen for a hydrogen gas supply.
Thus, an improved hydrogen generator is needed which is lightweight, compact, portable, and temperature safe and which provides a controllable flow of hydrogen upon demand.